1. Field of the Invention
This invention relates to laser gyroscopes.
2. The Prior Art
The laser gyroscope utilizes the properties of the optical oscillator (laser) and the theory of relativity to produce an integrating rate gyroscope. The laser gyroscope operates on a well-known principle that rotation of an operating ring laser (optical oscillator) about its axis causes the laser oscillation therein to experience an apparent change in path length for each direction. This change in path length causes a frequency shift in the oscillator. As between two counter-directionally travelling laser oscillations, this frequency shift results in the development of a beat frequency. This beat frequency is measured to provide an indication of the rate of angular rotation of the area circumscribed by the conventional ring laser oscillator or laser gyroscope.
The relationship between the observed beat frequency, .DELTA.f, and the rotation rate, .omega., is: EQU .DELTA.f = 4A.omega./.lambda.L (1)
where .lambda. is the wavelength of the laser radiation, A is the area enclosed or circumscribed by the ring laser and L is the length of the oscillator cavity. The oscillator cavity is defined as the optical path as determined by the optical components of the ring laser system.
The conventional laser gyroscope measures path differences of less than 10.sup.-6 A, and frequency changes of less than 0.1 Hertz, hereinafter, Hz (a precision of better than one part in 10.sup.15) in order to read rotation rates of less than 0.1.degree. per hour. The conventional laser gyroscope is simply a laser that has three or more reflective surfaces arranged to enclose an area. The three reflective surfaces, together with the light-amplifying material or gain medium in the laser path, forms the optical oscillator (laser). In fact, in the ring laser gyroscope there are two oscillators in the same physical cavity, one that has energy travelling clockwise and one that has energy travelling counter-clockwise. The frequency at which each oscillator operates is determined by the optical path length encountered by the laser radiation in the cavity in which it travels.
Apparent path length differences in the conventional ring laser gyroscope (wherein two oscillator paths are contained in essentially the same laser cavity and which length differences caused by rotation of the single cavity) create a shift in the frequencies in each of the two oscillators. On the other hand, physical changes in cavity length caused by temperature, vibrations, etc., do not cause frequency differences.
In order to sustain oscillation, two conditions must be met: (1) the gain within the oscillator must be equal to unity or greater at some power level set by the amplifying medium, and (2) the number of wave-lengths in the cavity must be an exact integer (that is, the phase shift around the cavity must be zero for each oscillator). If the first condition is to be achieved, the laser frequency must be such that the amplifying medium has sufficient gain to overcome the losses of the reflectors and other elements in the laser path.
In addition to the oscillator conditions of gain and loss, the condition of zero phase shift must also exist. Another way of stating this is that the number of wave-lengths in the cavity of the oscillator must be equal to an integer. In the laser oscillator this integer is in the millions and, therefore, a number of frequencies will satisfy these conditions of zero phase shift. However, these frequencies are separated by an amount equal to c/L (the speed of light, c, divided by the total length of the oscillator, L). For a total length of one meter the frequency separation is, therefore, 300 MHz.
Since the wavelength must be an exact integer for the laser radiation path around the cavity, it is this latter condition which actually determines the oscillation frequency of the oscillator. This condition results from the particular frequency (as determined by the path length) being the frequency which excites the gain medium to emit additional laser radiation at the frequency, hence it becomes an oscillator.
When the laser cavity is rotated, the clockwise and counterclockwise paths of the oscillator each have different apparent lengths. The path difference in these two directions causes the two oscillators to operate at different frequencies. The difference in the frequencies is proportional to the rate at which the ring is rotating since the apparent path length difference itself is proportional to the rotation rate. The readout of the laser gyroscope is accomplished by determining the frequency differential or, more particularly, beat frequency, between the two oscillators.
The fundamental condition is that the laser wave-length, .alpha., must be equal to an integer of the optical path length for the oscillator around the cavity. This integer is typically in the range of 10.sup.15 to 10.sup.7 (or larger, on certain geophysical applications) EQU L = N.lambda. (2)
accordingly, a change in length, .DELTA.L, will, correspondingly, cause a wavelength change, .DELTA..lambda., as follows: EQU .DELTA..lambda. = .DELTA.L/N (3)
the corresponding frequency change, .DELTA.f, is given as EQU .DELTA.f/f = .DELTA.L/L (4)
therefore, given small length differences, .DELTA.L, and reasonable cavity lengths, L, the operating frequency for a conventional ring laser gyroscope should be as high as possible.
The relationship between inertial input rates, .omega., and apparent length change .DELTA.L has been given as EQU .DELTA.L = 4A.omega./C (5)
the relationship between .DELTA.f and .omega., in terms of the gryroscope size and length is determined by substituting Equation 4 into Equation 5, giving EQU .DELTA.f = 4A.omega./.lambda.L (6)
where c = .lambda. f.
This concept forms the basis for recent developments in conventional laser gyroscopes wherein the apparent change in the length of the oscillator cavities for the ring laser manifests itself as a shift in the laser frequency and the development of a beat frequency between counter-directionally oscillating wavelengths. Beat frequency is, therefore, measurable to provide an indication of the rate of angular rotation of the oscillator cavity about an axis.
From the foregoing relationship, (Equation 6), it is readily observable that at extremely small angular rotation rates, .omega., the beat frequency, .DELTA.f, for that particular rotation rate, .omega., will also be relatively small.
Some of the limitations of the conventional ring laser gyroscope are phenomena known as "mode pulling" and "lock-in". These phenomena are experienced when the frequency difference between the two oscillators becomes small (less than about 500 Hz). Optical coupling between the two oscillators operating in essentially the same physical cavity pulls the frequencies closer together (mode pulling) and ultimately locks them together (lock-in) into one frequency, thereby eliminating any beat frequency (dead band) at low frequency differences.
When the output of the ring laser oscillator is observed as a function of the rotation rate it is readily seen that the beat frequency is directly proportional to the rotation rate for high rates of rotation. However, as the rotation rate decreases, the beat frequency falls to zero before the rotation rate falls to zero as a result of the foregoing phenomena of "lock-in". The rotation rate at which the observed beat frequency for a conventional ring laser gyroscope falls to zero depends upon the coupling between the two laser oscillation modes and there will always be coupling between oscillators in the same cavity. Obviously, therefore, the "lock-in" between the two frequencies of a single cavity, rotating ring laser oscillator cannot be reduced to zero.
Several techniques have been used to reduce the width of this "dead band" and increase the accuracy of the conventional ring laser gyroscope. These techniques include: (1) biasing the ring laser gyroscope by physically increasing the rate of rotation of the laser gyroscope (with a sinusoidally varying angular velocity, for example) and then subtracting out the induced biasing; or (2) introducing an optical element into the oscillator cavity, the optical element having an index of refraction dependent upon the direction of the laser radiation passing through the element. One of these latter phenomena is known as the Faraday Effect. However, these techniques introduce errors into the system and are also temperature dependent thereby greatly restricting the accuracy of the conventional single cavity ring laser gyroscope.
Alternatively, it has also been proposed to increase the size of the area, A, (see Equation 1) circumscribed by the laser cavity. However, increasing the size of the area circumscribed by the laser cavity has certain limitations as far as practical application of the laser gyroscope is concerned. These limitations include, for example, such factors as: accommodating the large gyroscope size in the vehicle in which it is placed and also temperature fluctuations experienced by the gyroscope's support structure. Other factors are changes caused by local support disturbances such as microseisms, etc. These latter factors are of significance since the laser gyroscope is also useful for the measurement of extremely small rotational rates and rotation rate changes, for example, those experienced in the measure of polar wobble, earth tides, continental drift, and length of day variations. Another factor is where the difference between rotation of a vehicle and the rotating earth become small such as in guidance systems, etc.
A useful discussion of some of the basic theories involved in the laser gyroscope may be found in IEEE SPECTRUM "The Laser Gyro", Joseph Killpatrick, October, pages 44-55 (1967).
In view of the foregoing, what is needed is an improved laser gyroscope in which the mode pulling and lock-in phenomena experienced in conventional laser gyroscopes at low rates of angular rotation does not exist. Such an improvement is disclosed in the present invention.